Specimens of rabbit liver were fixed for various periods up to 6 days in buffered14C-formaldehyde. Binding of the isotope reached a plateau after fixation for approximately 24 hr; the half-maximal binding level was reached after approximately 100 min. Formaldehyde binding at 37 C was faster than at 25 C, and faster at pH 7.0 than at pH 4.0. During rinsing of the fixed tissue in water for up to 26 days there was a progressive decrease in isotope content to 10–20% of the pre-rinse level, indicating that formaldehyde fixation is a reversible process.